Method for removing electrical discharge surface treatment film

Abstract

This method for removing a discharge surface treatment film (14) coated on the surface of a component (12) comprises a main washing step (S10) for performing main washing on a discharge surface treatment film (14) by using a main washing liquid containing a sodium permanganate and a primary alkali metal hydroxide, wherein the discharge surface treatment film (14) contains chromium.

Description

[Technical Field]

[0001] The present disclosure relates to a method for removing an electrical discharge surface treatment film, and more particularly to a method for removing an electrical discharge surface treatment film coated on the surface of a component. [Background technology]

[0002] Conventionally, the sliding surfaces of high-temperature parts such as turbine blades in aircraft gas turbine engines have been coated with electrical discharge surface treatment films made of hard metals or the like that have excellent durability and wear resistance. Electrical discharge surface treatment films are films formed by electrical discharge surface treatment. Electrical discharge surface treatment is a surface treatment technology that uses an electrode containing a coating component such as a hard metal to stably form a functional film that has excellent durability and wear resistance by applying electrical discharge energy (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2004 / 029329 Pamphlet Summary of the Invention [Problem to be solved by the invention]

[0004] When an electrical discharge surface treatment film coated on the surface of a part is thinned due to wear during operation of an aircraft jet engine or the like, the thinned electrical discharge surface treatment film is physically removed by mechanical polishing, and a new electrical discharge surface treatment film is applied. However, when the thinned electrical discharge surface treatment film is removed by mechanical polishing, there is a possibility that the part may be damaged during the mechanical polishing.

[0005] Therefore, an object of the present disclosure is to provide a method for removing an electrical discharge surface treatment film that can remove the electrical discharge surface treatment film while suppressing damage to a component coated with the electrical discharge surface treatment film. [Means for solving the problem]

[0006] The method for removing a discharge surface treatment film according to the present disclosure is a method for removing a discharge surface treatment film coated on the surface of a component, the discharge surface treatment film containing chromium, and includes a main cleaning step of main cleaning the discharge surface treatment film with a main cleaning solution containing sodium permanganate and a first alkali metal hydroxide.

[0007] The method for removing a discharge surface treatment film according to the present disclosure may include, before the main cleaning step, a pre-cleaning step of pre-cleaning the discharge surface treatment film with a pre-cleaning solution that contains a second alkali metal hydroxide, does not contain an oxidizing agent, and is more alkaline than the main cleaning solution.

[0008] In the method for removing a discharge surface treatment film according to the present disclosure, the pre-cleaning liquid may contain a surfactant.

[0009] The method for removing a discharge surface treatment film according to the present disclosure may include, after the main cleaning step, a post-cleaning step of post-cleaning the discharge surface treatment film with a post-cleaning solution that contains a third alkali metal hydroxide, does not contain an oxidizing agent, and is more alkaline than the main cleaning solution.

[0010] In the method for removing a discharge surface treatment film according to the present disclosure, the post-cleaning liquid may contain a surfactant.

[0011] The method for removing a discharge surface treatment film according to the present disclosure may include a pre-cleaning step of pre-cleaning the discharge surface treatment film with a pre-cleaning liquid containing a solvent.

[0012] In the method for removing a discharge surface treatment film according to the present disclosure, the preliminary cleaning liquid may contain a surfactant and an alkaline agent.

[0013] In the method for removing a discharge surface treatment film according to the present disclosure, the concentration of sodium permanganate contained in the cleaning solution may be 1% or more and 10% or less.

[0014] In the method for removing a discharge surface treatment film according to the present disclosure, the first alkali metal hydroxide may be sodium hydroxide or potassium hydroxide.

[0015] In the method for removing a discharge surface treatment film according to the present disclosure, the second alkali metal hydroxide may be sodium hydroxide or potassium hydroxide.

[0016] In the method for removing a discharge surface treatment film according to the present disclosure, the third alkali metal hydroxide may be sodium hydroxide or potassium hydroxide. [Effects of the Invention]

[0017] According to the above configuration, the electrical discharge surface treatment film can be removed while suppressing damage to the component coated with the electrical discharge surface treatment film. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a flowchart showing the configuration of a method for removing an electrical discharge surface treatment film in an embodiment of the present disclosure. [Figure 2] FIG. 2 is a cross-sectional view showing the configuration of a component provided with an electrical discharge surface treatment film in an embodiment of the present disclosure. [Figure 3A] FIG. 3A is a low-magnification overall photograph of an electrical discharge surface treatment coating, showing the results of cross-sectional observation of the metal structure of a test specimen before heat exposure in an embodiment of the present disclosure. [Figure 3B] FIG. 3B is a high-magnification photograph of an electrical discharge surface treatment coating showing a cross-sectional observation result of the metal structure of a test piece before heat exposure in an embodiment of the present disclosure. [Figure 4A] FIG. 4A is a low-magnification overall photograph of an electrical discharge surface treatment coating, showing the results of cross-sectional observation of the metal structure of a test specimen after heat exposure in an embodiment of the present disclosure. [Figure 4B] FIG. 4B is a high-magnification enlarged photograph of the electrical discharge surface treatment film, showing the cross-sectional observation results of the metal structure of the test piece after heat exposure in an embodiment of the present disclosure. [Figure 5A]FIG. 5A is a low-magnification overall photograph of an electrical discharge surface treatment film, showing the results of cross-sectional observation of the metal structure of a test piece that was subjected to the cleaning treatment of Example 1 in an embodiment of the present disclosure. [Figure 5B] FIG. 5B is a high-magnification enlarged photograph of the vicinity of the surface of the electrical discharge surface treatment film, showing the results of cross-sectional observation of the metal structure of a test piece that was subjected to the cleaning treatment of Example 1 in an embodiment of the present disclosure. [Figure 5C] FIG. 5C is a high-magnification enlarged photograph of the inside of the electrical discharge surface treatment film, showing the results of cross-sectional observation of the metal structure of a test piece that was subjected to the cleaning treatment of Example 1 in an embodiment of the present disclosure. [Figure 6A] FIG. 6A is a low-magnification overall photograph of an electrical discharge surface treatment film, showing the results of cross-sectional observation of the metal structure of a test piece that was subjected to the cleaning treatment of Comparative Example 1 in an embodiment of the present disclosure. [Figure 6B] FIG. 6B is a high-magnification enlarged photograph of the electrical discharge surface treatment film, showing the results of cross-sectional observation of the metal structure of a test piece that was subjected to the cleaning treatment of Comparative Example 1 in an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Fig. 1 is a flowchart showing the configuration of a method for removing a discharge surface treatment film. The method for removing a discharge surface treatment film is a method for removing a discharge surface treatment film coated on the surface of a component. First, the discharge surface treatment film will be described.

[0020] FIG. 2 is a cross-sectional view showing the configuration of a component 10 having an electrical discharge surface treatment coating. The component 10 has a component 12 and an electrical discharge surface treatment coating 14 coated on the surface of the component 12. The component 12 is, for example, a gas turbine component used in high-temperature environments, such as an aircraft jet engine component, a vehicle supercharger component, or an industrial gas turbine component. An example of an aircraft jet engine component is a turbine blade that slides against a shroud. The component 12 is formed of a heat-resistant alloy, such as a nickel alloy, a cobalt alloy, or an iron alloy.

[0021] The electrical discharge surface treatment film 14 is a film formed by electrical discharge surface treatment. First, electrical discharge surface treatment will be explained. Electrical discharge surface treatment is a surface treatment method in which powder of the material to be coated is solidified to form an electrode, which is then placed in insulating oil together with the component 12 and a voltage is applied to treat the surface. By repeatedly generating pulsed discharges between the electrode and the component 12, the electrode material migrates to the surface of the component 12 and melts and laminates, forming the electrical discharge surface treatment film 14.

[0022] The electrical discharge surface treatment film 14 contains chromium (Cr). When the electrical discharge surface treatment film 14 is exposed to a high-temperature oxidizing atmosphere, the chromium contained in the electrical discharge surface treatment film 14 is selectively oxidized to form an oxide film containing chromium oxide (Cr2O3). This oxide film functions as a protective oxide film with excellent oxidation resistance. Furthermore, chromium oxide (Cr2O3) functions as a high-temperature lubricant, thereby improving wear resistance.

[0023] The electrical discharge surface treatment film 14 can be formed, for example, from a cobalt alloy containing cobalt (Co) as the main component and also containing chromium (Cr) and silicon (Si). The main component in an alloy is the alloy component that accounts for the largest proportion in the alloy. Examples of the cobalt alloy that can be used include Stellite alloy and Tribaloy alloy.

[0024] Stellite alloys are cobalt alloys containing Cr, Si, W, C, etc., with the remainder being Co and unavoidable impurities. Stellite alloys, for example, contain cobalt as the main component, 20% by mass to 32.5% by mass of Cr, and 2.0% by mass or less of Si, and have excellent heat resistance and oxidation resistance. Stellite alloys are hard and have excellent wear resistance because fine carbides such as WC are dispersed in them. For example, Stellite 31 alloy can be used as the Stellite alloy.

[0025] Triballoy alloys are cobalt alloys containing Cr, Si, Mo, etc., with the remainder being Co and unavoidable impurities. Triballoy alloys, for example, contain cobalt as the main component, 8.5% by mass to 18% by mass of Cr, and 1.3% by mass to 3.7% by mass of Si, and have excellent heat resistance and oxidation resistance. Triballoy alloys are hard and have excellent wear resistance because fine intermetallic compounds of Mo and Si are dispersed in them. Triballoy alloys include Triballoy T-400 alloy, T-800 alloy, etc.

[0026] The electrical discharge surface treatment film 14 has a porous metal structure because it is formed by repeatedly applying pulsed discharges between the electrode and the component 12, causing the electrode material to migrate to the component 12 and melt and build up. The thickness of the electrical discharge surface treatment film 14 can be, for example, 5 μm to 3000 μm. When the electrical discharge surface treatment film 14 is exposed to a high-temperature environment, chromium contained in the electrical discharge surface treatment film 14 is oxidized to form chromium oxide (Cr2O3) on the surface and in the pores of the electrical discharge surface treatment film 14.

[0027] After operation of an actual aircraft such as a jet engine, the electrical discharge surface treatment film 14 tends to thin due to wear. If a new film is formed by electrical discharge surface treatment to make up for the lost thickness, a boundary will form between the electrical discharge surface treatment film 14 after heat exposure and the newly formed electrical discharge surface treatment film 14, making the electrical discharge surface treatment film 14 prone to peeling. For this reason, the electrical discharge surface treatment film 14 after heat exposure is removed, and then a new electrical discharge surface treatment film 14 is formed.

[0028] Next, returning to FIG. 1 , a method for removing the discharge surface treatment film 14 coated on the surface of the component 12 will be specifically described. The method for removing the discharge surface treatment film 14 includes a main cleaning step (S10). The method for removing the discharge surface treatment film 14 may include a pre-cleaning step (S12) before the main cleaning step (S10). The method for removing the discharge surface treatment film 14 may include a post-cleaning step (S14) after the main cleaning step (S10). The method for removing the discharge surface treatment film 14 may include a pre-cleaning step (S12) before the main cleaning step (S10) and a post-cleaning step (S14) after the main cleaning step (S10). Each step will be described in detail below.

[0029] The main cleaning step (S10) is a step in which the discharge surface treatment film 14 is main cleaned with a main cleaning solution containing sodium permanganate and a first alkali metal hydroxide. In the main cleaning step (S10), the chromium contained in the discharge surface treatment film 14 is actively oxidized to produce chromium oxide (Cr2O3), which is an amphoteric oxide. Then, by dissolving the produced chromium oxide (Cr2O3), the metal structure of the discharge surface treatment film 14 can be made more porous.

[0030] This cleaning solution contains sodium permanganate and a first alkali metal hydroxide. This cleaning solution may further contain a surfactant or the like. This cleaning solution can contain sodium permanganate and a first alkali metal hydroxide, with the balance being a solvent. This cleaning solution may contain sodium permanganate, a first alkali metal hydroxide, and a surfactant, with the balance being a solvent. The solvent for this cleaning solution may be, for example, water.

[0031] Sodium permanganate functions as an oxidizing agent that oxidizes the chromium contained in the discharge surface treatment film 14. Sodium permanganate can promote the formation of chromium oxide (Cr2O3) by oxidizing the chromium contained in the discharge surface treatment film 14. Commercially available sodium permanganate can be used.

[0032] The concentration of sodium permanganate in this cleaning solution can be 1% to 10%, preferably 3% to 7%. If the concentration of sodium permanganate is lower than 1%, the formation of chromium oxide (Cr2O3) may be reduced. If the concentration of sodium permanganate is 10%, chromium oxide (Cr2O3) can be sufficiently formed.

[0033] The first alkali metal hydroxide functions as an alkaline agent that dissolves chromium oxide (Cr2O3). Because chromium oxide (Cr2O3) is an amphoteric oxide, chromium oxide (Cr2O3) can be dissolved with an alkaline agent. The first alkali metal hydroxide can dissolve chromium oxide (Cr2O3) formed when chromium contained in the discharge surface treatment film 14 is oxidized by sodium permanganate. The first alkali metal hydroxide can also dissolve chromium oxide (Cr2O3) contained in the oxide film formed on the surface of the discharge surface treatment film 14 during heat exposure. Furthermore, the first alkali metal hydroxide can dissolve chromium oxide (Cr2O3) formed in the pores of the discharge surface treatment film 14 during heat exposure. This allows the metal structure of the discharge surface treatment film 14 to become more porous.

[0034] The primary alkali metal hydroxide is preferably sodium hydroxide or potassium hydroxide. Sodium hydroxide and potassium hydroxide are strong alkaline agents, which improve the solubility of chromium oxide (Cr2O3). Commercially available sodium hydroxide or potassium hydroxide can be used.

[0035] The concentration of the first alkali metal hydroxide in this cleaning solution can be 10% or more and 20% or less, and is preferably 14%. This is because if the concentration of the first alkali metal hydroxide is lower than 10%, the solubility of chromium oxide (Cr2O3) decreases. If the concentration of the first alkali metal hydroxide is 20%, chromium oxide (Cr2O3) can be sufficiently dissolved.

[0036] The surfactant may be, for example, an anionic surfactant, a nonionic surfactant, or the like. The anionic surfactant may be, for example, a fatty acid surfactant, an alkylbenzene surfactant, a higher alcohol surfactant, or an α-olefin surfactant. The nonionic surfactant may be, for example, a fatty acid surfactant, a higher alcohol surfactant, or an alkylphenol surfactant. The surfactant may be, for example, a linear alkylbenzene sulfonate, a polyoxyethylene alkyl ether sulfate, or a poly(oxyethylene) nonylphenyl ether. When a surfactant is added to the cleaning solution, the concentration of the surfactant in the cleaning solution should be greater than 0% and less than or equal to 15%.

[0037] The main cleaning can be performed, for example, by immersing the discharge surface treatment film 14 in the main cleaning solution. The temperature of the main cleaning solution can be, for example, room temperature. The main cleaning solution may also be heated before use. The immersion time in the main cleaning solution can be, for example, 60 to 120 minutes. After the main cleaning of the discharge surface treatment film 14, it is recommended to rinse with water to remove the main cleaning solution. Note that the main cleaning is not limited to immersion, and other cleaning methods such as spray, shower, and jet may also be used.

[0038] By performing the main cleaning of the discharge surface treatment film 14, the main cleaning solution penetrates into the pores of the discharge surface treatment film 14. The sodium permanganate in the main cleaning solution actively oxidizes the chromium contained in the discharge surface treatment film 14 to form chromium oxide (Cr2O3). The first alkali metal hydroxide in the main cleaning solution dissolves the formed chromium oxide (Cr2O3). This makes the metal structure of the discharge surface treatment film 14 more porous, making it easier to peel off and remove the discharge surface treatment film 14.

[0039] Furthermore, the metal structure of the component 12 is denser than the metal structure of the electrical discharge surface treatment film 14, which prevents the cleaning solution from penetrating into the component 12. This prevents damage to the component 12 not only when the component 12 is made of a heat-resistant alloy that does not contain chromium, but also when the component 12 is made of a heat-resistant alloy that contains chromium.

[0040] The method for removing the discharge surface treatment film 14 may include a pre-cleaning step (S12) before the main cleaning step (S10). The pre-cleaning step (S12) is a step of pre-cleaning the discharge surface treatment film 14 before the main cleaning step (S10) with a pre-cleaning solution that contains a second alkali metal hydroxide, does not contain an oxidizing agent, and is more alkaline than the main cleaning solution.

[0041] The pre-cleaning step (S12) mainly dissolves and removes chromium oxide (Cr2O3), an amphoteric oxide contained in the oxide film formed on the surface of the electrical-discharge surface treatment film 14. This makes it easier for the main cleaning solution to penetrate into the electrical-discharge surface treatment film 14 in the main cleaning step (S10).

[0042] The pre-cleaning liquid contains a second alkali metal hydroxide. The pre-cleaning liquid may further contain a surfactant or the like. The pre-cleaning liquid can contain a second alkali metal hydroxide, with the remainder being a solvent. The pre-cleaning liquid may contain a second alkali metal hydroxide and a surfactant, with the remainder being a solvent. The solvent for the pre-cleaning liquid may be, for example, water. The pre-cleaning liquid does not contain an oxidizing agent. This is because the pre-cleaning step (S12) mainly dissolves chromium oxide (Cr2O3) contained in the oxide film formed on the surface of the discharge surface treatment film 14, and therefore there is no need to actively oxidize the chromium contained in the discharge surface treatment film 14.

[0043] The second alkali metal hydroxide functions as an alkaline agent that dissolves chromium oxide (Cr2O3), an amphoteric oxide. The second alkali metal hydroxide is preferably sodium hydroxide or potassium hydroxide. The second alkali metal hydroxide dissolves the chromium oxide (Cr2O3) contained in the oxide film formed on the surface of the discharge surface treatment film 14 during heat exposure. This removes the oxide film formed on the surface of the discharge surface treatment film 14, making it easier for the cleaning solution to penetrate into the discharge surface treatment film 14 in the main cleaning step (S10).

[0044] The concentration of the second alkali metal hydroxide in the pre-cleaning solution can be 40% or more and 50% or less, and is preferably 41%. This is because a concentration of the second alkali metal hydroxide of 40% or more and 50% or less can sufficiently dissolve chromium oxide (Cr2O3) contained in the oxide film formed on the surface of the discharge surface treatment film 14 during heat exposure.

[0045] The pre-cleaning liquid is more alkaline than the main cleaning liquid. This improves the solubility of chromium oxide (Cr2O3) contained in the dense oxide film formed on the surface of the electrical-discharge surface treatment film 14. For example, the second alkali metal hydroxide in the pre-cleaning liquid may be an alkali metal hydroxide that is more alkaline than the first alkali metal hydroxide in the main cleaning liquid. Furthermore, when the second alkali metal hydroxide in the pre-cleaning liquid and the first alkali metal hydroxide in the main cleaning liquid are the same alkali metal hydroxide, the concentration of the second alkali metal hydroxide in the pre-cleaning liquid may be higher than the concentration of the first alkali metal hydroxide in the main cleaning liquid.

[0046] The surfactant may be, for example, an anionic surfactant or a nonionic surfactant. For example, linear alkylbenzene sulfonate, polyoxyethylene alkyl ether sulfate, poly(oxyethylene) nonylphenyl ether, or the like. The surfactant in the pre-cleaning solution may be the same as or different from the surfactant in the main cleaning solution. When a surfactant is added to the pre-cleaning solution, the concentration of the surfactant in the pre-cleaning solution may be greater than 0% and not more than 0.5%, and is preferably not more than 0.1%.

[0047] The pre-cleaning can be performed, for example, by immersing the discharge surface treatment film 14 in a pre-cleaning solution. The temperature of the pre-cleaning solution can be, for example, room temperature. The pre-cleaning solution may be heated before use. The immersion time in the pre-cleaning solution can be, for example, 120 to 180 minutes. After the discharge surface treatment film 14 has been pre-cleaned, it is recommended to rinse with water to remove the pre-cleaning solution. Note that the pre-cleaning is not limited to immersion, and other cleaning methods such as spraying, showering, and jetting may also be used.

[0048] By pre-cleaning the discharge surface treatment film 14, the second alkali metal hydroxide in the pre-cleaning solution dissolves chromium oxide (Cr2O3) contained in the oxide film formed on the surface of the discharge surface treatment film 14 during heat exposure. This removes the oxide film formed on the surface of the discharge surface treatment film 14, which makes it easier for the main cleaning solution to penetrate into the discharge surface treatment film 14 in the main cleaning step (S10), thereby facilitating the main cleaning.

[0049] The method for removing the discharge surface treatment film 14 may include a post-cleaning step (S14) after the main cleaning step (S10). The post-cleaning step (S14) is a step of post-cleaning the discharge surface treatment film 14 after the main cleaning step (S10) with a post-cleaning solution that contains a third alkali metal hydroxide, does not contain an oxidizing agent, and is more alkaline than the main cleaning solution.

[0050] In the post-cleaning step (S14), even if chromium oxide (Cr2O3) remains in the electrical discharge surface treatment film 14 after the main cleaning step (S10), the remaining chromium oxide (Cr2O3) can be dissolved and removed.

[0051] The post-cleaning liquid contains a third alkali metal hydroxide. The post-cleaning liquid may further contain a surfactant or the like. The post-cleaning liquid can contain a third alkali metal hydroxide, with the remainder being a solvent. The post-cleaning liquid can contain a third alkali metal hydroxide and a surfactant, with the remainder being a solvent. The solvent is preferably water, for example. The post-cleaning liquid does not contain an oxidizing agent. This is because the post-cleaning step (S14) mainly dissolves and removes chromium oxide (Cr2O3) remaining in the electrical discharge surface treatment film 14 after the main cleaning step (S10). The post-cleaning liquid may be the same as the pre-cleaning liquid, or a different cleaning liquid may be used.

[0052] The third alkali metal hydroxide functions as an alkaline agent that dissolves chromium oxide (Cr2O3), which is an amphoteric oxide. The third alkali metal hydroxide is preferably sodium hydroxide or potassium hydroxide. The third alkali metal hydroxide can dissolve and remove chromium oxide (Cr2O3) remaining in the electrical discharge surface treatment film 14 after the main cleaning step (S10).

[0053] The concentration of the tertiary alkali metal hydroxide in the post-cleaning solution can be 40% or more and 50% or less, and is preferably 41%. If the concentration of the tertiary alkali metal hydroxide is 40% or more and 50% or less, chromium oxide (Cr2O3) remaining in the electrical discharge surface treatment film 14 can be sufficiently dissolved.

[0054] The post-cleaning liquid is more alkaline than the main cleaning liquid. This improves the solubility of chromium oxide (Cr2O3) remaining in the electrical discharge surface treatment film 14. For example, the third alkali metal hydroxide in the post-cleaning liquid may be an alkali metal hydroxide that is more alkaline than the first alkali metal hydroxide in the main cleaning liquid. When the third alkali metal hydroxide in the post-cleaning liquid and the first alkali metal hydroxide in the main cleaning liquid are the same alkali metal hydroxide, the concentration of the third alkali metal hydroxide in the post-cleaning liquid may be higher than the concentration of the first alkali metal hydroxide in the main cleaning liquid.

[0055] The surfactant may be, for example, an anionic surfactant or a nonionic surfactant. For example, linear alkylbenzene sulfonate, polyoxyethylene alkyl ether sulfate, poly(oxyethylene) nonylphenyl ether, or the like. The surfactant in the post-cleaning solution may be the same as or different from the surfactant in the main cleaning solution or the pre-cleaning solution. When a surfactant is added to the post-cleaning solution, the concentration of the surfactant in the post-cleaning solution may be greater than 0% and not more than 0.5%, and is preferably not more than 0.1%.

[0056] The post-cleaning can be performed by, for example, immersing the discharge surface treatment film 14 in a post-cleaning solution. The temperature of the post-cleaning solution can be, for example, room temperature. The post-cleaning solution may be heated before use. The immersion time in the post-cleaning solution can be, for example, 60 to 120 minutes. After the discharge surface treatment film 14 has been post-cleaned, it is recommended to rinse with water to remove the post-cleaning solution. Note that the post-cleaning is not limited to immersion, and other cleaning methods such as spraying, showering, and jetting may also be used.

[0057] By post-cleaning the discharge surface treatment film 14, the post-cleaning solution penetrates into the discharge surface treatment film 14, and the tertiary alkali metal hydroxide contained in the post-cleaning solution dissolves the chromium oxide (Cr2O3) remaining in the discharge surface treatment film 14. This promotes the porosity of the metal structure of the discharge surface treatment film 14.

[0058] The method for removing the discharge surface treatment film 14 may include a pre-cleaning step of pre-cleaning the discharge surface treatment film 14. The pre-cleaning step is a step in which the discharge surface treatment film 14 is pre-cleaned in advance with a pre-cleaning solution containing a solvent. The pre-cleaning step mainly removes oil and the like adhering to the discharge surface treatment film 14. If the pre-cleaning step (S12) is performed, the pre-cleaning step can be performed before the pre-cleaning step (S12). Furthermore, if the pre-cleaning step (S12) is not performed, the pre-cleaning step can be performed before the main cleaning step (S10).

[0059] The preliminary cleaning liquid contains a solvent. The preliminary cleaning liquid may further contain a surfactant, an alkaline agent, etc. The preliminary cleaning liquid can contain a solvent, with the remainder being a solvent. The preliminary cleaning liquid may contain a solvent, a surfactant, and an alkaline agent, with the remainder being a solvent. The solvent may be, for example, water.

[0060] The solvent has the function of removing oil and other contaminants adhering to the discharge surface treatment film 14. For example, 2-(2-butoxyethoxy)ethanol can be used as the solvent. The concentration of the solvent in the preliminary cleaning solution can be 1% or more and 10% or less, and preferably 3% or more and 8% or less. This is because a solvent concentration of 1% or more and 10% or less can sufficiently remove oil and other contaminants adhering to the discharge surface treatment film 14.

[0061] The surfactant may be, for example, an anionic surfactant or a nonionic surfactant. For example, linear alkylbenzene sulfonate, polyoxyethylene alkyl ether sulfate, poly(oxyethylene) nonylphenyl ether, or the like. The surfactant in the preliminary cleaning solution may be the same as or different from the surfactants in the main cleaning solution, pre-cleaning solution, and post-cleaning solution. When a surfactant is added to the preliminary cleaning solution, the concentration of the surfactant in the preliminary cleaning solution may be 5% or more and 20% or less, and preferably 10% or more and 15% or less.

[0062] The alkaline agent may be, for example, ammonia, etc. When an alkaline agent is added to the preliminary cleaning liquid, the concentration of the alkaline agent in the preliminary cleaning liquid may be 0% or more and 0.1% or less, and is preferably less than 0.07%.

[0063] The preliminary cleaning can be performed, for example, by immersing the discharge surface treatment film 14 in a preliminary cleaning solution. The temperature of the preliminary cleaning solution can be, for example, room temperature. The preliminary cleaning solution may be heated before use. The immersion time in the preliminary cleaning solution can be, for example, 60 to 120 minutes. After the discharge surface treatment film 14 has been pre-cleaned, it is recommended to rinse with water to remove the preliminary cleaning solution. Note that the preliminary cleaning is not limited to immersion, and other cleaning methods such as spraying, showering, and jetting may also be used.

[0064] The method for removing the electrical discharge surface treatment film 14 may involve performing the main cleaning step (S10) once, or may involve repeatedly performing the main cleaning step (S10). When a preliminary cleaning step is performed, it may be performed once before the first main cleaning step (S10), even if the main cleaning step (S10) is repeated. After all steps are completed, the product may be dried.

[0065] The method for removing the electrical discharge surface treatment film 14 may involve one cycle of the pre-cleaning step (S12) and the main cleaning step (S10), or multiple cycles of the pre-cleaning step (S12) and the main cleaning step (S10). When a preliminary cleaning step is performed, it is sufficient to perform it once before the first pre-cleaning step (S12), even when multiple cycles of the pre-cleaning step (S12) and the main cleaning step (S10) are performed. After all steps are completed, the product should be dried.

[0066] The method for removing the electrical discharge surface treatment film 14 may involve one cycle of the main cleaning step (S10) and the post-cleaning step (S14), or multiple cycles of the main cleaning step (S10) and the post-cleaning step (S14). When a preliminary cleaning step is performed, it is sufficient to perform it once before the first main cleaning step (S10), even when multiple cycles of the main cleaning step (S10) and the post-cleaning step (S14) are performed. After all steps are completed, the product should be dried.

[0067] The method for removing the electrical discharge surface treatment film 14 may involve one cycle of the pre-cleaning step (S12), the main cleaning step (S10), and the post-cleaning step (S14), or multiple cycles of the pre-cleaning step (S12), the main cleaning step (S10), and the post-cleaning step (S14). When a preliminary cleaning step is performed, even if multiple cycles of the pre-cleaning step (S12), the main cleaning step (S10), and the post-cleaning step (S14) are performed, it is sufficient to perform the preliminary cleaning step once before the first pre-cleaning step (S12). After all steps are completed, the product should be dried.

[0068] As described above, the above-described configuration includes a main cleaning step, allowing the discharge surface treatment film to be main cleaned with a main cleaning solution containing sodium permanganate and a first alkali metal hydroxide. This allows the chromium contained in the discharge surface treatment film to be oxidized with sodium permanganate to form the amphoteric oxide chromium oxide (Cr2O3), which can then be dissolved with the first alkali metal hydroxide. As a result, the discharge surface treatment film becomes more porous, allowing it to be easily peeled off and removed.

[0069] According to the above configuration, since a pre-cleaning step is provided before the main cleaning step, the discharge surface treatment film can be pre-cleaned with a pre-cleaning solution containing a second alkali metal hydroxide. As a result, even if an oxide film containing chromium oxide (Cr2O3) is formed on the surface of the discharge surface treatment film, the chromium oxide (Cr2O3) can be dissolved with the second alkali metal hydroxide to remove the oxide film.

[0070] According to the above configuration, since the post-cleaning step is provided after the main cleaning step, the discharge surface treatment film can be post-cleaned with a post-cleaning solution containing a third alkali metal hydroxide. As a result, even if chromium oxide (Cr2O3) remains on the discharge surface treatment film, the remaining chromium oxide (Cr2O3) can be dissolved and removed with the third alkali metal hydroxide.

[0071] According to the above configuration, since the pre-cleaning step is provided, the discharge surface treatment film can be pre-cleaned in advance with a pre-cleaning solution containing a solvent, so that even if oil or the like is attached to the surface of the discharge surface treatment film, the oil or the like can be removed with the solvent.

[0072] According to the above configuration, the discharge surface treatment film coated on the surface of the component is dissolved and removed using a cleaning fluid such as the present cleaning fluid, so the discharge surface treatment film can be removed while minimizing damage to the component compared to when the discharge surface treatment film is physically removed by mechanical polishing. Furthermore, according to the above configuration, the component has a metal structure that is denser than the discharge surface treatment film, so penetration of the cleaning fluid such as the present cleaning fluid into the component is suppressed. This makes it possible to remove the discharge surface treatment film while minimizing damage to the component, such as corrosion. [Example]

[0073] The discharge surface treatment film coated on the surface of the substrate was subjected to a cleaning treatment, and the peelability of the discharge surface treatment film was evaluated.

[0074] First, the test specimen will be described. The test specimen was prepared by coating the surface of a substrate with an electrical discharge surface treatment film. The substrate was formed of a Ni alloy. The electrical discharge surface treatment film was formed of a Stellite 31 alloy. The alloy composition of the Stellite 31 alloy was, by mass, 9.5% to 11.5% Ni, 2.0% or less Fe, 0.45% to 0.55% C, 24.5% to 26.5% Cr, 1.0% Mn, 1.0% Si, and 7.5% W, with the remainder being Co and unavoidable impurities.

[0075] Next, a method for applying an electrical discharge surface treatment film will be described. First, an electrode for electrical discharge surface treatment was produced using Stellite 31 alloy powder. The Stellite 31 alloy powder used consisted of large particle size powder with an average particle size of 8 μm or less and small particle size powder with a particle size of 3 μm or less. The large particle size powder, small particle size powder, binder, and lubricant were mixed to produce granulated powder. The granulated powder was compression-molded to form a green compact, which was then fired to produce an electrode.

[0076] The electrode and substrate were placed in insulating oil, and a pulsed discharge was generated between the electrode and substrate using a discharge power supply. This discharge energy was used to adhere the electrode material to the surface of the substrate, forming a discharge surface treatment film. The thickness of the discharge surface treatment film was approximately 500 μm. To simulate the operation of the actual equipment, the test specimen was exposed to heat in an air atmosphere at 750°C for 100 hours.

[0077] Next, the heat-exposed test specimens were subjected to the cleaning treatments of Example 1 and Comparative Example 1 to evaluate the peelability of the electrical discharge surface treatment film. In the cleaning treatments of Example 1 and Comparative Example 1, the same test specimens were used.

[0078] First, we will explain the cleaning process of Example 1. In the cleaning process of Example 1, after preliminary cleaning, pre-cleaning, main cleaning, and post-cleaning were performed, pre-cleaning, main cleaning, and post-cleaning were further performed, and finally, the test specimen was dried.

[0079] A pre-cleaning solution was used for the pre-cleaning. The pre-cleaning solution contained 3% to 8% solvent, 10% to 15% surfactant, and less than 0.07% alkaline agent, with the remainder being water. 2-(2-butoxyethoxy)ethanol was used as the solvent. Poly(oxyethylene) nonylphenyl ether was used as the surfactant. Ammonia was used as the alkaline agent. For the pre-cleaning, the specimen was immersed in the pre-cleaning solution for 60 minutes and then rinsed with water.

[0080] For pre-cleaning, a pre-cleaning solution was used. The pre-cleaning solution contained 41% alkali metal hydroxide, 0.1% or less surfactant, and the remainder was water. Sodium hydroxide was used as the alkali metal hydroxide. For pre-cleaning, the specimen was immersed in the pre-cleaning solution for 120 minutes, and then rinsed with water.

[0081] This cleaning solution was used for this cleaning. This cleaning solution contained 14% alkali metal hydroxide, 3% to 7% sodium permanganate, and the remainder was water. Sodium hydroxide was used as the alkali metal hydroxide. For this cleaning, the test specimen was immersed in this cleaning solution for 60 minutes, and then rinsed with water.

[0082] For post-cleaning, a post-cleaning solution was used. The post-cleaning solution contained 41% alkali metal hydroxide, 0.1% or less surfactant, and the remainder was water. Sodium hydroxide was used as the alkali metal hydroxide. For post-cleaning, the specimen was immersed in the post-cleaning solution for 60 minutes, and then rinsed with water.

[0083] Next, the cleaning process of Comparative Example 1 will be described. In the cleaning process of Comparative Example 1, only the preliminary cleaning and pre-cleaning in the cleaning process of Example 1 were performed, and then the test specimen was finally dried. In the cleaning process of Comparative Example 1, the main cleaning and post-cleaning in the cleaning process of Example 1 were not performed.

[0084] Next, cross-sectional observation of the metal structure of the test specimen was performed. The cross-sectional observation of the metal structure was performed using an optical microscope. First, the results of the cross-sectional observation of the metal structure of the test specimen before heat exposure will be described. Figure 3 is a photograph showing the cross-sectional observation of the metal structure of the test specimen before heat exposure. Figure 3A is a low-magnification overall photograph of the discharge surface treatment film, and Figure 3B is a high-magnification enlarged photograph of the discharge surface treatment film. Note that arrow A in Figure 3B indicates pores in the discharge surface treatment film. The discharge surface treatment film had many pores and was composed of a porous metal structure. On the other hand, the substrate was composed of a denser metal structure than the discharge surface treatment film.

[0085] Next, we will explain the results of cross-sectional observation of the metal structure of the test specimen after heat exposure. Figure 4 is a photograph showing the results of cross-sectional observation of the metal structure of the test specimen after heat exposure. Figure 4A is a low-magnification overall photograph of the discharge surface treatment film, and Figure 4B is a high-magnification enlarged photograph of the discharge surface treatment film. In the test specimen after heat exposure, an oxide film containing chromium oxide (Cr2O3) was formed on the surface of the discharge surface treatment film. Chromium oxide (Cr2O3) was also formed in the pores of the discharge surface treatment film. Chromium oxide (Cr2O3) is thought to have been formed by the oxidation of chromium contained in the discharge surface treatment film due to heat exposure. Note that arrow B in Figure 4B indicates chromium oxide (Cr2O3) formed in the pores of the discharge surface treatment film.

[0086] Next, we will explain the results of cross-sectional observation of the metal structure of the test specimen that underwent the cleaning treatment of Example 1. Figure 5 is a set of photographs showing the results of cross-sectional observation of the metal structure of the test specimen that underwent the cleaning treatment of Example 1. Figure 5A is a low-magnification overall photograph of the electrical discharge surface treatment coating, Figure 5B is a high-magnification enlarged photograph of the vicinity of the surface of the electrical discharge surface treatment coating, and Figure 5C is a high-magnification enlarged photograph of the interior of the electrical discharge surface treatment coating.

[0087] The oxide film that had formed on the surface of the discharge treatment film after heat exposure was removed from the specimen that had undergone the cleaning treatment of Example 1. The discharge surface treatment film of the specimen that had undergone the cleaning treatment of Example 1 had a more porous metal structure than the discharge surface treatment film before or after heat exposure. The metal structure near the surface of the discharge surface treatment film was more porous than the interior of the discharge surface treatment film. This made the discharge surface treatment film easier to peel off, and it was found that the discharge surface treatment film could be removed.

[0088] Furthermore, damage such as corrosion caused by the pre-cleaning solution, main cleaning solution, post-cleaning solution, and preliminary cleaning solution was suppressed for the substrate, which is thought to be mainly due to the fact that the substrate is composed of a denser metal structure than the discharge surface treatment film.

[0089] Next, we will explain the results of cross-sectional observation of the metal structure of the test piece that was subjected to the cleaning treatment of Comparative Example 1. Figure 6 is a photograph showing the results of cross-sectional observation of the metal structure of the test piece that was subjected to the cleaning treatment of Comparative Example 1, where Figure 6A is a low-magnification overall photograph of the electrical discharge surface treatment coating, and Figure 6B is a high-magnification enlarged photograph of the electrical discharge surface treatment coating.

[0090] In the specimen that underwent the cleaning treatment of Comparative Example 1, the oxide film that had formed on the surface of the discharge treatment film after heat exposure was removed. However, the discharge treatment film of the specimen that underwent the cleaning treatment of Comparative Example 1 exhibited a metal structure that was approximately the same as the discharge surface treatment film before and after heat exposure. In other words, the degree of porosity of the discharge surface treatment film of the specimen that underwent the cleaning treatment of Comparative Example 1 was approximately the same as the degree of porosity of the discharge surface treatment film before and after heat exposure. This shows that the discharge surface treatment film was difficult to peel off and remove from the specimen that underwent the cleaning treatment of Comparative Example 1.

[0091] The weight change was measured for the specimen that had undergone the cleaning treatment of Example 1. The weight of the specimen before heat exposure was 22.2920 g. The weight of the specimen after heat exposure was 22.3189 g. The weight of the specimen that had undergone the cleaning treatment of Example 1 was 22.2780 g. The weight of the specimen that had undergone the cleaning treatment of Example 1 was 0.0409 g lighter than the specimen after heat exposure. This indicates that the chromium contained in the electrical discharge surface treatment film had dissolved as chromium oxide (Cr2O3), an amphoteric oxide.

[0092] The entire contents of Patent Application No. 2022-070110 (filing date: April 21, 2022) are incorporated herein by reference. [Explanation of symbols]

[0093] 10 Parts with discharge surface treatment film 12 parts 14 Electric discharge surface treatment film

Claims

1. A method for removing an electrical discharge surface treatment film coated on a surface of a component, comprising: the electrical discharge surface treatment coating contains chromium, a main cleaning step of main cleaning the discharge surface treatment film with a main cleaning solution containing sodium permanganate and a first alkali metal hydroxide; The method for removing a discharge surface treatment film includes, before the main cleaning step, a pre-cleaning step of pre-cleaning the discharge surface treatment film with a pre-cleaning solution that contains a second alkali metal hydroxide, does not contain an oxidizing agent, and is more alkaline than the main cleaning solution.

2. The method for removing an electrical discharge surface treatment film according to claim 1, The method for removing a discharge surface treatment film, wherein the pre-cleaning liquid contains a surfactant.

3. A method for removing an electrical discharge surface treatment film coated on the surface of a part, comprising: the electrical discharge surface treatment coating contains chromium, a main cleaning step of main cleaning the discharge surface treatment film with a main cleaning solution containing sodium permanganate and a first alkali metal hydroxide; The method for removing a discharge surface treatment film includes, after the main cleaning step, a post-cleaning step of post-cleaning the discharge surface treatment film with a post-cleaning solution that contains a third alkali metal hydroxide, does not contain an oxidizing agent, and is more alkaline than the main cleaning solution.

4. The method for removing an electrical discharge surface treatment film according to claim 3, The method for removing a discharge surface treatment film, wherein the post-cleaning liquid contains a surfactant.

5. The method for removing an electrical discharge surface treatment film according to claim 1, The method for removing a discharge surface treatment film comprises a pre-cleaning step of pre-cleaning the discharge surface treatment film with a pre-cleaning solution containing a solvent.

6. The method for removing an electrical discharge surface treatment film according to claim 5, The method for removing a discharge surface treatment film, wherein the preliminary cleaning liquid contains a surfactant and an alkaline agent.

7. The method for removing an electrical discharge surface treatment film according to claim 1, The method for removing a discharge surface treatment film, wherein the concentration of sodium permanganate contained in the cleaning solution is 1% or more and 10% or less.

8. The method for removing an electrical discharge surface treatment film according to claim 1, The method for removing a discharge surface treatment film, wherein the first alkali metal hydroxide is sodium hydroxide or potassium hydroxide.

9. The method for removing an electrical discharge surface treatment film according to claim 1, The method for removing a discharge surface treatment film, wherein the second alkali metal hydroxide is sodium hydroxide or potassium hydroxide.

10. The method for removing an electrical discharge surface treatment film according to claim 3, The method for removing a discharge surface treatment film, wherein the tertiary alkali metal hydroxide is sodium hydroxide or potassium hydroxide.

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